Photo: Alien prop in museum

Millions of people believe in aliens like this prop in the International UFO Museum and Research Center in Roswell, New Mexico. There is no conclusive evidence of any life beyond Earth, but there is hope: If microbes can live in the pores of rock deep beneath the Earth or at the rim of a scalding Yellowstone spring, then they might find a place like Mars not so shabby.

Photograph by Peter Essick

Written by Joel Achenbach

Republished from the pages of National Geographic magazine

Something astonishing has happened in the universe. There has arisen a thing called life—flamboyant, rambunctious, gregarious form of matter, qualitatively different from rocks, gas, and dust, yet made of the same stuff, the same humdrum elements lying around everywhere.

Life has a way of being obvious—it literally scampers by, or growls, or curls up on the windowsill—and yet it's notoriously difficult to define in absolute terms. We say that life replicates. Life uses energy. Life adapts. Some forms of life have developed large central processing networks. In at least one instance, life has become profoundly self-aware.

And that kind of life has a big question: What else is alive out there?

There may be no scientific mystery so tantalizing at the brink of the new millennium and yet so resistant to an answer. Extraterrestrial life represents an enormous gap in our knowledge of nature. With instruments such as the Hubble Space Telescope, scientists have discovered a bewildering amount of cosmic turf, and yet they still know of only a single inhabited world.

We all have our suppositions, our scenarios. The late astronomer Carl Sagan estimated that there are a million technological civilizations in our galaxy alone. His more conservative colleague Frank Drake offers the number 10,000. John Oro, a pioneering comet researcher, calculates that the Milky Way is sprinkled with a hundred civilizations. And finally there are skeptics like Ben Zuckerman, an astronomer at UCLA, who thinks we may as well be alone in this galaxy if not in the universe.

All the estimates are highly speculative. The fact is that there is no conclusive evidence of any life beyond Earth. Absence of evidence is not evidence of absence, as various pundits have wisely noted. But still we don't have any solid knowledge about a single alien microbe, a solitary spore, much less the hubcap from a passing alien starship.

Our ideas about extraterrestrial life are what Sagan called "plausibility arguments," usually shot through with unknowns, hunches, ideologies, and random ought-to-bes. Even if we convince ourselves that there must be life out there, we confront a second problem, which is that we don't know anything about that life. We don't know how truly alien it is. We don't know if it's built on a foundation of carbon atoms. We don't know if it requires a liquid-water medium, if it swims or flies or burrows.

Despite the enveloping nebula of uncertainties, extraterrestrial life has become an increasingly exciting area of scientific inquiry. The field is called exobiology or astrobiology or bioastronomy—every few years it seems as though the name has been changed to protect the ignorant.

Whatever it's called, this is a science infused with optimism. We now know that the universe may be aswarm with planets. Since 1995 astronomers have detected at least 22 planets orbiting other stars. NASA hopes to build a telescope called the Terrestrial Planet Finder to search for Earth-like planets, examining them for the atmospheric signatures of a living world. In the past decade organisms have been found thriving on our own planet in bizarre, hostile environments. If microbes can live in the pores of rock deep beneath the earth or at the rim of a scalding Yellowstone spring, then they might find a place like Mars not so shabby.

Mars is in the midst of a full-scale invasion from Earth, from polar landers to global surveyors to rovers looking for fossils. A canister of Mars rocks will be rocketed back to Earth in the year 2008, parachuting into the Utah desert for scrutiny by scientists in a carefully sealed lab. In the coming years probes will also go around and, at some point, into Jupiter's moon Europa. That icy world shows numerous signs of having a subsurface ocean—and could conceivably harbor a dark, cold biosphere.

The quest for an alien microbe is supplemented by a continuing effort to find something large, intelligent, and communicative. SETI—the Search for Extraterrestrial Intelligence—has not yielded a confirmed signal from an alien civilization in 40 years of experiments, but the signal-processing technology grows more sophisticated each year. The optimists figure it's only a matter of time before we tune in the right channel.

No one knows when—or if—one of these investigations might make a breakthrough. There's a fair bit of boosterism surrounding the entire field, but I'd bet the breakthrough is many years, if not decades, away. The simple truth: Extraterrestrial life, by definition, is not conveniently located.

But there are other truths that sustain the search for alien organisms. One is that, roughly speaking, the universe looks habitable. Another is that life radiates information about itself—that, if nothing else, it usually leaves a residue, an imprint, an echo. If the universe contains an abundance of life, that life is not likely to remain forever in the realm of the unknown.

Contact with an alien civilization would be an epochal and culturally challenging event, but exobiologists would settle gladly for the discovery of a tiny fossil, a mere remnant of extraterrestrial biochemistry. One example. One data point to add to the one we have—Earth life. That's what we need to begin the long process of putting human existence in its true cosmic context.

Exobiologists go to the worst places on Earth, or at least the most extreme—the driest, coldest, most Mars-like or Europa-like environments they can find.

If you want to track down the exobiologist Jack Farmer from Arizona State University, you look for him in Death Valley, on the shores of nearby Mono Lake, or swimming beneath the ice shelf in Antarctica. If seeking Chris McKay, you might check out the Atacama Desert of Chile or some island north of the Arctic Circle.

The place to find Penny Boston is in the nastiest cave imaginable. I tagged along with Boston on one of her trips to a wet, bat-ridden cave in southern Mexico called Villa Luz. Boston has been studying the microbes that thrive there—in environments where a human being not wearing a gas mask would perish.

"All my life I've wanted to cross the cosmos, go to other plants," says Boston. "This is probably as close as I'll get at my age."

Boston and her friend Diana Northrup, a librarian and cave biologist in New Mexico, are undeterred by the face-smashing gas masks they must wear or by the constant wetness, the darkness, the bats, or the slight possibility that a belch of carbon monoxide would kill everyone. Not are they overly concerned about the various threats of malaria and dengue fever and whatever other exotic diseases they might pick up here. Before we entered Villa Luz, I asked if other was any danger of encountering an unknown, Ebola-like pathogen. "We think it's moderately unlikely," Boston said.

The cave floor was covered with water of varying depths and no transparency, and we walked gingerly so as to avoid discovering unmapped deep water. By caving standards, though, this was a walk in the park—no ropes required, just some crawling and scrambling through low-ceilinged passages.

Eventually we reached the deepest, largest chamber, known as the Great Hall. Midges flitted, spiders spun webs, bats zagged and zigged just over our heads, emitting their high-pitched sonar. Red rock walls were covered with green slime, black muck, gooey white gypsum paste, and limestone in the process of being dissolved by sulfuric acid.

Just as I was thinking how much this cave resembled the human nasal cavity, we came to the snottites (Boston is lobbying to have the word recognized as a scientific term). Snottites are gelatinous structures formed by microbial wastes. They dangle from the ceiling. Boston and her team have been measuring their growth, trying to understand the metabolism of the microbes and their long-term effect on the geology of the cave. Dry weather since her last visit seemed to have inhibited the growth of the structures.

Mike Spilde, another member of the team, splashed over to where I'd been inspecting a water bug whose shell was covered with eggs. He reached into a spring burbling from under a rock and pulled out some gray wads the consistency ff cooked cabbage. These are known, in keeping with the theme of the place, as phlegm balls. They are vibrant microbial communities, not clinging to life in a narrow niche but proliferating in it, replicating up a storm.

Taking a break back on the surface, Boston placed some of her cave work in context.

"We have discovered"—she means scientists in general—"organisms thriving in environments harsh to us but essential to them. It broadens your perspective. We all suffer to some extent from 'expertitis' in science. It's good for your soul, and good for your intellect, and good for your work to have your imagination stretched, to be open to the possibilities."

The most tantalizing possibility is that the universe hums with life and that in the coming centuries we will find it. An exobiologist's abiding optimism is fired by the knowledge that living things are primarily constructed of hydrogen, nitrogen, carbon, and oxygen—the four most common chemically active elements in the universe. And life is inextricably interwoven with nonlife; not even the sharpest razor can perfectly slice them apart.

We also know that a functioning ecosystem does not require sunlight or photosynthesis. In the early 1990s researchers found that the basaltic rock deep beneath Washington State contains an abundance of microbes totally cut off from the photosynthetic world. Even more complex life can adapt to hostile places. When scientists in the deep-sea submersible Alvin went tooling around the mid-ocean ridges, they found hot vents covered with shrimp and mouthless tube worms.

What remains unknown is whether life can survive over time in narrow ecological niches on largely barren worlds. Could life survive in aquifers far below the harsh surface of Mars? What could ensure the cold, dark environment of Europa's hypothesized ocean? Can an alien world have just a little bit of life, or are biospheres and all-or-nothing proposition?

The cave at Villa Luz, as remote as it is, does not exist in isolation. It is a small, connected piece of a world that riots with life.

As scientists struggle to find a trace of life somewhere else in the universe, there exists for many people a more dramatic situation, one in which extraterrestrial life isn't microbial and slimy but rather intelligent, technological, and lurking in our midst. The believers in these aliens are not likely to be convinced that ETs are a bogus phenomenon. An ability to elude detection and confirmation, particularly by mainstream thinkers, is a presumed characteristic of the Visitors.

Having dropped in on a couple of UFO conventions and visited Roswell, New Mexico and its UFO museum, I've come to the conclusion that it's not possible to wing an argument about space aliens. True believers and skeptics rarely go over to the other side. I think it's fair to say, however, that flying-saucer aliens lack scientific stature. If they insist on being so jumpy, if they insist on abducting people in the middle of the night when no one else can verify their presence, they have no right to enter a reputable natural history museum.

But neither are people who believe in the UFO narrative—which generally is dated to the 1947 sighting of some flying "disks" near Mount Rainier in Washington State—necessarily irrational, much less crazy, as they are sometimes depicted. Most people operate from the same instinct, which is to know the truth about the universe. That so many people would adopt a theory of aliens utterly contrary to that of mainstream science (and that of, among agencies, the U.S. Air Force, which spent 22 years investigating UFO reports) is a reminder of the special attraction of extraterrestrial life.

As many writers have noted, aliens are, for some people, the secular equivalent of angels and demons and ghostly spirits. The aliens are an extrapolation of modern astronomy and engineering (big universe, fast rocket ships), but they also possess some ancient urge to come to Earth and mess with human beings. What makes them so intriguing is that even scientists will concede that alien beings could very well be out there somewhere. Therefore the scenario in which they some to Earth requires only some imagination about transportation.

Many scientists don't wonder why aliens are buzzing the Earth in flying saucers—they wonder why they aren't. In 1950 Enrico Fermi, physicist, asked some of his colleagues a question that would become famous: Where is everybody? Humans could theoretically colonize the galaxy in a million years or so, and if they could, astronauts from older civilizations could do the same. So why haven't they come to Earth? This is known as the Fermi paradox.

Could it be that they're observing us but not interfering? (The zoo hypothesis.) Did they come and leave artifacts and get bored and go away? (This is the "ancient astronauts" idea that posits the aliens as builders of pyramids and so forth." Or could it be that for all intelligent species, interstellar travel is too expensive and time-consuming? (It's just less than 25 trillion miles [40.2 trillion kilometers] from Earth to the nearest stars beyond the sun.)

Or could it be possible that, at least in our part of the galaxy, the most technologically advanced species is the one right here on Earth?

Our contemporary culture did not invent this idea of life beyond Earth. The alien is a Hollywood stock character but not a Hollywood creation. More than 2,000 years ago the Greek philosopher Metrodorus of Chios wrote, "It is unnatural in a large field to have only one shaft of wheat, and in the infinite Universe only one living world." Four centuries ago Giordano Bruno was burned at the stake in part because he believed that there were inhabited worlds throughout the cosmos. Astronomers like Christian Huygens supplemented their purely scientific work with treatises on the characteristics of life beyond Earth. Huygens felt, for example, that aliens would probably have hands, like humans.

Missing from the debate, typically, was the one ingredient of a truly persuasive argument: Evidence. That seemed to change with the apparent discovery of the Martian canals. In 1877 Giovanni Schiaparelli, an Italian astronomer, found what he called canali, or channels, on the surface of the planet. The American astronomer Percival Lowell and a few colleagues took the idea from there.

In the final years of the 9th century, Lowell, using a new telescope he built near Flagstaff, Arizona, revealed the discovery of hundreds of canals and argued that these were the artificial creations of an intelligent Martian civilization. In fact, he wrote, the Martians would certainly have to be superior to us. He reasoned that their globe-spanning engineering projects were far beyond our own capabilities and that the ability of a race of creatures to live in harmony over the whole of a planet showed them to be of a more advanced character than our own squabbling selves. H.G. Wells tweaked the idea just a bit in his novel The War of the Worlds, in which Martians come to Earth with deadly heat rays and dreams of conquest.

The Martians, alas, were doomed, except as cultural artifacts. When astronomers looked at Mars with more powerful telescopes, there were no canals anywhere. Lowell's canals were created in his mind's eye—a classic example of the saying "Believing is seeing." But there remained, into the 1960s, a fascination with waves of seasonal darkening on the surface. Could this be vegetation? The Martian prairies and forests were conclusively eradicated in 1965, when the Mariner 4 probe took 22 pictures of the surface. Mars was a cratered wasteland, reminiscent of the moon.

When the Viking landers descended to the Martian surface in 1976, they found no compelling sign of life and indeed discovered that the surface contains no trace of organic molecules. Though the mission was a fantastic triumph of science and technology, the absence of detectable life on Mars put exobiology in a two-decade funk.

The mood changed in the 1990s. Biologists were detecting organisms in such exotic environments on Earth that they were inspired to look anew at the rest of the solar system as potentially habitable. They also discovered signs that life appeared early in the Earth's history. Intriguingly, at about the time life arose on Earth, Mars was a much more hospitable planet than it is today. Images of the Martian surface indicate that the planet once had flowing rivers and perhaps an ocean. Life could even have started on Mars and spread to Earth aboard a meteorite.

Which brings up the most famous Martian meteorite: ALH84001. In 1996 a team of three NASA scientists based in Houston announced that this potato-size rock, found in Antarctica, contained what appeared to be Martian fossils.

The discovery was proclaimed at an unforgettable NASA press conference in Washington, D.C., on August 7, 1996. Everyone realized the historical glory of being right about these purported microfossils—and the reciprocal tarnish of being wrong. Dan Goldin, NASA Administrator, cautioned that the results were not definitive, but he said, "We may see the first evidence that life might have existed beyond the confines of this planet, the third rock from the sun."

The NASA team made a dramatic presentation, complete with graphics and the first, startling images of the microfossils, one of which looked like a worm (others a bit like Cheetos). But then came a dissenter, UCLA's J. William Schopf, who said that on a scale of one to ten of increasing probability of biological origin, he could only grant the alleged Martian fossils a two. So began, that day, an enduringly divisive scientific debate.

The NASA scientists had to admit that their four primary lines of evidence could each be explained nonbiologically. They had found, for example, PAHs, polycyclic aromatic hydrocarbons, which sometimes are associated with living things but which can also be found in car exhaust. They found grains of magnetite, which might have been produced inside microbes or might not have. In a sense the research raised the question of whether a series of possibilities add up to a probability. At the least it runs headlong into a Sagan dictum, which is that extraordinary claims require extraordinary evidence.

The NASA team saw its conclusions vigorously attacked. One damaging study showed that some of the microbe-like structures were merely flakes of the rock rendered more biological in appearance by the coating process used in the preparation of slides. Researchers also found contaminants from Earth inside the meteorite.

The team fought these challenges point by point, but after three years critics felt they'd pretty much killed off the Mars rock. Luann Becker, a geochemist at the University of Hawaii, told me, "I think we're beating a dead horse."

But Everett Gibson, part of the NASA meteorite team, sees this as a typical scientific resistance to a revolutionary idea. "Science," he said, "doesn't accept radical ideas quickly."

There was a time when scientists didn't believe that meteorites could possibly fall from the sky. There was a time when plate tectonics—the movement and collision and subduction of vast slabs of the Earth's crust—was deemed a very strange idea. Are the Mars rock fossils in the same category? Or are they more like those canals?

If life sprang up through natural processes on the Earth, then the same thing could presumably happen on other worlds. And yet when we look at outer space, we do not see an environment teeming with life. We see planets and moons where no life as we know it could possibly survive. In fact we see all sorts of wildly different planets and moons—hot place, murky places, ice worlds, gas worlds—and it seems that there are far more ways to be a dead world than a live one.

Within our solar system the Earth may be in a fairly narrow habitable zone, not too hot and not too cold, just the right distance from the sun that water can splash around on the surface in a liquid state. And there may be many other things that make life on Earth possible. The tectonic activity recycles the planet's carbon. Mars has no such mechanism, and this seemingly minor deficiency may be the reason Mars lost most of its atmosphere.

The search for extraterrestrial life is in some ways a search for constraints, for the things that limit the emergence of life or the evolution of complex organisms. For calculating the number of technological, communicative civilizations, the most popular theoretical tool is the Drake equation.

In 1960 an American astronomer named Frank Drake became the first person to conduct a sensitive radio search for signals from extraterrestrial civilizations. He aimed an 85-foot (26-meter) radio telescope at two nearby stars and, after one false alarm, found no intentional signals. The next year, preparing for a meeting of visionary thinkers (including the young Sagan), he made an outline for how to discuss the probably of detecting intelligent life, starting with the rate of star formation and the typical number of planets and working through to the longevity of civilizations. "I thought it was just a gimmick. It's amazing to me now that it's in the astronomy textbooks," he told me.

Going through the factors from left to right—N=R*fpneflfifcL—you don't get very far before you hit some serious unknowns. Jill Tarter, who has dedicated her career to SETI, says, "The Drake equation is a wonderful way to organize our ignorance."

The only factor well understood, R*, tells us the number of stars. Suffice it to say that there are lots of them, more than a hundred billion in our galaxy alone, maybe as many as 400 billion (and that doesn't count, of course, the billions of other galaxies). The second factor, fp, the fraction of stars with planets, is rapidly coming into focus. There are still uncertainties, since the detection equipment can find only extremely massive planets. These behemoths aren't like the Earth. Many of the extrasolar planets discovered so far may have migrated toward the parent star over time, destroying any rocky, Earth-like planets along the way.

Eventually, the Terrestrial Planet Finder (TPF) could help solve the next factor in the equation, ne—the number of planes with habitable environments—and may even be able to glean some evidence of the following factor, fl—the fraction on which life has originated. TPF, still many years from construction, would capture the feeble reflected light from a distant rocky planet, while nulling the far more brilliant light of the parent star. This remnant of light might amount to only a single pixel of data. The light could then be examined for the spectral signature of, for example, oxygen, methane, ozone, or some indicator of a planet with biological processes. Thrilling as such a discovery would be, it's easy to imagine how it would echo the situation with the Mars rock. There would likely be no "proof" of life, merely an interpretation subject to much second-guessing.

Even on Earth the origin of life is a stubbornly enduring mystery. "How can a collection of chemicals form themselves into a living thing without any interference from outside?" asks Paul Davies, a physicist and writer. "On the face of it, life is an exceedingly unlikely event," he argues. "There is no known principle of matter that says it has to organize itself into life. I'm very happy to believe in my head that we live in a biofriendly universe, because in my heart I find that very congenial. But we have not yet discovered the Life Principle."

No one is even sure that life requires liquid water, though that seems a reasonable bet and is surely the case on Earth. Liquid water may be fairly scarce in the universe—Europa may help solve that issue—but another presumed ingredient of life, organic molecules, those made up primarily of carbon, are commonplace. That's why Jeffrey Bada, a pretty hard-nosed researcher, thinks the universe is full of living things. "I don't see any way to avoid that," he said, sounding almost apologetic.

So let's assume that life can spring up in many places. Now comes fi, another giant unknown in the Drake equation: How often does life evolve to a condition of intelligence?

There are those, like Ernst Mayr, one of the great biologists of the 20th century, who argue that high intelligence has occurred only once on Earth, among something like a billion species. Hence it is a billion-to-one long shot. But Paul Horowitz, a Harvard physicist, argues that the same data can be looked at the opposite way: That on the only planet we know of that has life, intelligence appeared. That's a one-for-one proposition.

I've never met anyone who thinks that if you rewound the tape of terrestrial evolution (to use Stephen Jay Gould's metaphor) and played it again, you'd wind up with a genetically identical human being the second time around. But there are those who say that an intelligent being is more likely under certain initial conditions. The paleobiologist Andy Knoll argues that intelligence is rooted in the emergence of structures that allow simple animals to sense their environment and seek food. "If we get to creepy crawlies that look for food, then at some point intelligent life may emerge," he says.

There are those who argue passionately that alien life would be nothing like us—in Fred Hoyle's novel The Black Cloud the alien is a gaseous cloud that decides to feed on our sun—and there are others who say the biology of the Earth is probably a pretty good example of what's out there.

Finding life somewhere else, even a single alien amoeba, might clarify the extent to which life evolves along parallel tracks—and whether it typically arrives at certain useful structures, such as eyeballs, wings, and large brains. Human beings have, by far, the biggest brains on Earth in ratio to body size. Did we get these things in our skulls through a random, improbable evolutionary quirk?

Lori Marino, a psychobiologist at Emory University, points out that dolphins appear to have undergone a dramatic increase in brain size in the past 35 million years, which may have a parallel in the quadrupling of brain size among hominids in the past few million years. By her reckoning, huge leaps in intelligence may be found among creatures on worlds everywhere in the universe.

But it's also true that the data are scarce, and this is still a territory for, among others, philosophers and theologians. What does it mean to be "intelligent"? When we "think" or "feel" or "love," what is it that we are doing? When we ask if we are "alone," we really want to know if there are others out there in the universe who are, in key aspects, very much like ourselves. We seek the communicators,—Drake's fc, creatures who have the technology to send signals—storytellers, ideally.

Every three years a bioastronomy meeting gathers many of the leading thinkers in the field. I went to the 1999 assemblage in August on the Big Island of Hawaii, and at the opening reception around a hotel pool a University of Toronto sociologist named Allen Tough offered a provocative theory:

"I think a probe is already here. It's probably been here a long time."

He didn't mean flying saucers. His alien probes would be much smaller—"nanoprobes," tiny robotic exploratory craft sent to Earth from advanced civilizations. The alien probes may, at some point, let themselves be known to human civilization. How? Where? "I think it will happen on the World Wide Web," said Tough.

Tough and about a dozen other visionaries had a pre-conference meeting to discuss what to do if human civilization receives a "high-content" message from extraterrestrials. There was much uncertainty about how well prepared humankind is for such an event. We might have trouble crafting a response. Should we be forthcoming about the flaws of our species? If we acknowledge our history of wars and slavery, could that be misinterpreted as a threat? What if, even as an international committee of well-meaning thinkers tried to put together a message, some guerilla radio broadcaster or "shock jock" beat everyone to it?

Bioastronomy also has its more down-to-Earth side. The meeting reminded me how much there is still to learn about out little solar system. Exobiologist Jack Farmer made a simple yet stunning point one morning when he noted that neither the Viking landers in 1976 nor the Pathfinder spacecraft in 1997 carried to Mars the tool so vital to a geologist: a magnifying lens. Nor would the polar lander scheduled for a December 1999 landing carry such an instrument. Farmer's comment remained in my mind when Cindy Lee Van Dover, an oceanographer, noted that no one has ever made a dive in a deep-sea submersible to an active hot vent in the Indian Ocean to see what might be alive down there.

So before we worry about our dealings with the Galactic Empire, we have some serious fieldwork to do closer to home.

Freeman Dyson, a physicist, has argued that humans may engineer new forms of life that will be adapted to living in the vacuum of space or on the surface of frozen moons and comets and asteroids. In Dyson's universe, life is mobile, and planets are gravitational traps inhibiting free movement.

"Perhaps our destiny is to be the midwives, to help the living universe to be born," he said recently. "Once life escapes from this little planet, there'll be no stopping it."

But life must first survive this planet. The longevity of civilizations is the final factor in the Drake equation, the haunting letter L. Humans in their modern anatomy have been around only 125,000 years or so. It is not clear yet that a brain like ours is necessarily a long-term advantage. We make mistakes. We build bombs. We ravage our world, poison its water, foul its air. Our first order of business, as a species, is to make L as long an interval as possible.

I would hope that anyone who investigates this issue will come away with a renewed appreciation of what and who we are. In a universe of empty space and stellar furnaces and ice worlds, it is good to be alive. And we should remember that even if we find intelligent life beyond Earth, it may not be what we expect or even what we were searching for.

The alien may not speak to that part of our consciousness that we deem most important—our spirit, if you will. It may have little to teach us. The great moment of contact may simply remind us that what we most want is to find a better version of ourselves—a creature we will probably have to make, from our own raw elements, here on Earth.

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